text:
What type of pilot, if any, can use the legal title 'Captain'?
I came across a YouTube channel of a pilot who calls himself 'Captain [name]'... this guy is a Microlight pilot. It made me cringe. Anyway, it also got me wondering, what type of license, if any, allows someone to use the title 'Captain' legally?
Clarification: I mean on official documentation, such as your name and title on a 'details' form, such as at a Doctor's office.*

It's not a legal question.

It may depends on the country you are looking for, and if you are speaking as a "role" or as a "rank". And do not forget that "Captain" is not exclusive to aviation.

Possible duplicate, although it answers a slightly different question: https://aviation.stackexchange.com/questions/58812/what-exactly-do-captain-pilot-co-pilot-and-first-officer-mean

hey @Jamiec thanks for the link, that looks like a very different question to me but I appreciate it :)

Have added a bit of clarity to the question

Another possible duplicate: https://aviation.stackexchange.com/questions/63837/is-it-required-to-be-an-atp-and-work-for-an-airline-to-be-a-captain/63843#63843

Questions about using unearned titles, (that have broad meaning outside aviation) on doctor office forms are definitely more legal than aviation.  I’m not saying we should close or move because it also fits here, just offering a counterpoint to @GdD’s unconditional proclamation.

@757toga, those guys must have been full of hot air!  Have you see the "Maestro" episode of Seinfeld?

Does this answer your question? Is it required to be an ATP and work for an airline to be a Captain?

So Captain Crunch might be lying ?

germany
"Captain" (or its German equivalent "Kapitän") is not a protected title. Everybody and anybody can call themselves "Captain" for any reason or no reason under any circumstance without any type of license required.
It is, of course, possible to sign a contract that regulates how you are allowed to use that title within the context laid out in said contract. For example, many airlines uses the title "Kapitän" to refer a pilot who has achieved a certain rank within said airline. So, if an airline pilot who has not attained the rank of Kapitän calls themselves a "Kapitän" within the context of their employment as a pilot of said airline, then this might be a violation of their employment contract and thus they might face disciplinary action. But this is a private contractual matter between said pilot and said airline, not a law. [Note: I don't know what an actual airline employment contract looks like and whether airlines actually enforce this.]
If, OTOH, that same pilot rents a sailboat and takes the rudder, or plays football and is elected as the Kapitän of his team, or rides a tandem with a friend, they can refer to themselves as a "Kapitän", and any articles of their employment contract that would prohibit this would be void.

It might be worth adding that misusing certain protected titles (e.g. Dr.) is a criminal offence in Germany (§ 132a StGB). "Captain" or "Kapitän" just isn't listed as protected (neither is "pilot" under protected job designations).

The general form "Captain" is defined as

Captain is a title, an appellative for the commanding officer of a military unit; the supreme leader of a navy ship, merchant ship, aeroplane, spacecraft, or other vessel.

"Captain" is also a rank bestowed by one's employer when working on the flight deck of a commercial operation.
More generally, a "pilot in command" is the person who is ultimately responsible for the safe operation of an aircraft and in that respect when flying as a private pilot as PIC, you are the "pilot in command". More colloquially, this is informally referred to as the "Captain" and hence us PPL-types love to call ourselves "Captain [Surname]" as it makes us feel important!

With regard to your edit; no, it is not a title that you should put on an official or unofficial form of any sort. Where it is an option for "title", that would presumably be reserved by a military rank of the same name

Cheers Jamie! So is it a 'Courtesy title' as described here? https://en.wikipedia.org/wiki/Courtesy_titles_in_the_United_Kingdom

@Cloud It's not much different than "driver" for a car, "rider" for a bike, or, well, also "captain" for a boat. With the exception that cars don't have a "second driver".

@Cloud I wouldn't call it a courtesy title, no. That seems to be specifically to do with nobility.

The title of Captain can officially be used by someone whom flys a multi-pilot aircraft (aeroplane or helicopter) that is the holder of an ATPL(A/H) (Airline Transportation Pilots License), and is the Pilot In Command (PIC) of the flight and qualified to the rank of Captain in a multi-pilot certified aircraft type and operation. The second pilot is First Officer (second in command) or Senior First Officer.
It absolutely is an official title, recognised by Europe (EASA) the UK (UK CAA) and the US (FAA) as well as many other ICAO state members.
In the case of a single pilot aircraft, many confuse this classification of Captain when in fact it is legally only ‘Pilot In Command’ but not strictly Captain as there is no other flight crew to define rank against.
A Captain, in the correct term is the PIC but the PIC is not necessarily a Captain.

How about some citations to the relevant regulations?


text:
Can motorjets be a viable means of propulsion?
A motorjet is a very early type of jet engine where intake air passes through a compressor driven by a separate piston engine, and is then mixed with fuel and burned to provide thrust. They were used in a handful of designs around the time of World War II, but fell out of favour when gas turbine metallurgy had advanced to the point where it was feasible to drive the engine compressor from a turbine in the exhaust stream without having to rebuild the engine every few flights; all else being equal, a motorjet, requiring an entire second engine to drive the compressor, is heavier, and, thus, has a lower thrust-to-mass ratio than a turbojet or turbofan. Are there any situations where a motorjet would still be a better choice than a turbojet or turbofan?
I can think of a few maybes:

Aircraft using fuel that produces refractory oxides when burned (such as boron-containing fuels and most organometallics), or operating in air heavily contaminated with particles of same (such as volcanic ash clouds), as these are very hard on turbines unless the exhaust gas temperature (EGT) remains high enough to vapourise these oxides all the way out the tailpipe; a motorjet has no turbine blades in the exhaust stream to suffer ill effects from this (unlike a turbojet or turbofan), does not need an onboard supply of oxidiser (unlike a rocket), and produces reasonable thrust even at zero airspeed (unlike a ramjet).
Aircraft with very severe length restrictions, as a turbojet or turbofan's compressor(s) and turbine(s) all have to be in line with one another, whereas a motorjet's compressor motor could be stacked vertically above the compressor and combustion chamber.
VTOL aircraft, where the torque and gyroscopic effects from all that heavy, rapidly-spinning turbomachinery can cause control problems while hovering or moving at very low speed (due to the control authority of aerodynamic control surfaces being very low at very low airspeeds, and nil at zero airspeed), as pointed out in this comment by Peter Kämpf on an answer to one of my earlier questions.

https://en.wikipedia.org/wiki/Liquid_air_cycle_engine uses similar solution and I guess would qualify as a modern version of a motorjet? So potentially of interest despite not being really relevant to the question, I hope.

A motorjet is not a useful configuration.
The purpose of a motorjet is to generate pure jet propulsion without the difficulties associated with use of a turbine. The elimination of the turbine is not the material factor in determining whether this configuration still has a useful purpose. The important thing to identify is whether or not pure jet propulsion is useful.
The trend in aviation propulsion is towards ever higher bypass ratios, due to the increased fuel efficiency of a fan or prop compared to that of a pure jet. This means ever more power is mechanically extracted from the core to drive a fan or prop.

An internal combustion engine already has all its useful power available in mechanical form, which can then be used to drive a prop or fan. Diverting energy to drive the compressor and combustor in a motorjet is exactly opposite of the trend towards fuel efficiency,  making it useless in subsonic applications.
The ICE is also a very heavy substitute for a turbine, so even supersonic applications that benefit from low bypass operation won't use it either.

isn't motorjet infinite bypass ratio by definition?

@user3528438 A motorjet has a zero bypass ratio. All the air goes through the combustor where it is heated to produce jet thrust. There is no power extraction to drive a fan or prop.

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text:
Why aren't any jet fuels heavier than Jet A/A-1/JP-8 in common use?
The commonly-used jet fuels fall into two main categories:

Straight kerosenes (Jet A, Jet A-1, JP-8, JP-5, plus, historically, JP-1), which are used in most situations because their high flashpoints make them safer to handle.
Gasoline-kerosene blends (Jet B, plus, historically, JP-3 and JP-4), which are used in very cold climates due to their lower freezing points, but are much more dangerous to handle due to their very low flashpoints.

However, even the straight kerosenes have flashpoints below 40C, which can easily be exceeded in hot-weather operations, potentially with lethal results.  Using a fuel with a somewhat heavier composition (and, thus, a higher flashpoint) would greatly alleviate this danger; for instance, the lighter diesels have flashpoints in the 50C-60C range, and would, thus, seem to be a safer choice than kerosene-based fuels for hot-weather operations (especially in tropical and subtropical climates where very hot weather is more common than elsewhere), where their higher freezing points would not be an issue.
So why aren't there any "heavy" jet fuels for hot-weather use?

diesel is kind of expensive

Yeah but when you go from the surface in the tropics to 40000 ft, it's still gonna be -40 or -50, so gel resistance is still the priority.

@user3528438, diesel is cheaper than kerosene. Generally, the heavier the fraction, the cheaper it is: at least, 'objectively'. What makes diesel expensive in some areas is taxation and market peculiarities. But if all aviation flew on diesel the market would be very different.

Wide cut were used specifically for low flash-point starting performance, not the freezing point.
Flameout was common in early jet engines and the designs were very hard to re-start while at high altitude, the gasoline fraction was just much easier to ignite in a cold condition.

@MaxPower: Originally, yes.  Nowadays, where wide-cut fuels are still used, it's for their low freezing point.

@Vikki Where is jet-B still used? There are very cold places that use A1, but A1 isn't wide cut.

@MaxPower: IIRC, Alaska and northern Canada.

As @john K said, fuel needs to be liquid at cruising altitude. The less volatile a hydrocarbon is at a temperature, the quicker it will stop flowing as the temperature drops.
Additionally, the plane is not necessarily landing in the same hot climate it is taking off from. You need a fuel that is stable across the range of temperatures experienced while operating.
This post addresses the specific behavior of different fuels in great depth.

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text:
In general, how do aircraft handle differently in inverted flight versus in upright flight?
Inverted flight is flight with a roll angle between 90° and 270° (although it classically referred specifically to flight with a roll angle of 180° - i.e., upside-down).  In what ways, in general, does aircraft handling in inverted flight differ from aircraft handling in upright (non-inverted) flight?
Partially inspired by reading about the crash of Alaska Airlines Flight 261, including how the pilots apparently tried (albeit unsuccessfully) to recover their airplane from its final dive by rolling it inverted.

You may want to limit your question to a specific airframe (MD-83 perhaps based on your example) or perhaps a general group of aircraft. This question is fairly broad and some aircraft are designed to work well inverted while others may not be making a broad answer tough to pin down.

It would certainly matter whether the aircraft is negatively-loaded or positively-loaded.  Either would be possible.

One factor in any type of flying is airspeed. 250 knots indicated would be an EF 5 tornado.  Most objects that are not attached to earth will fly in any configuration, upright, inverted, even on their sides.
The original X1 that broke the sound barrier had (by today's standards) amazingly glider-like wings.
Reducing them allowed for even higher speeds.
Inverted flight limitations and concerns are mainly pilot training, fuel flow, trim, and objects falling to the ceiling.  Notice that weight forward and elevator "up" will cause a rapid descent when inverted, which is corrected by trimming elevator "down" (is the new up).
The aircraft, retrimmed, will fly, although less efficiently.  A fully symmetrical airfoil (common on aerobatic aircraft) minimizes these differences.  Stall speeds will vary with aircraft in inverted flight, but can be avoided.  The aircraft, even Flight 261, would have a workable flight envelope.
Sadly, in the case of Flight 261, they were dealing with a jammed and then completely detached horizontal stabilizer (allegedly poor design and improperly maintained) and simply had no chance once it broke free.  Had the plane been in proper working order, I would not doubt those 2 experienced pilots, with permission, could have flown inverted quite easily.
One would expect less roll stability from the anhedral, but more from the fuselage and tail (weight) being below the wing.  Shame they could not save it.

As an aside, re roll stability, wing sweep contributes a stabilizing effect in negatively-loaded flight as well as in positively-loaded flight; it's only at 0 G's that this effect vanishes.  That could be incorporated into the last paragraph of this answer.

Typically, unless the CG is quite far aft (as is often the case with aircraft designed for aerobatics), lots of forward stick or yoke will be needed to keep the nose high enough for sustained inverted flight. With inadequate forward yoke or stick input, the airspeed may increase very quickly as the flight path curves earthward and altitude is rapidly lost.
This orange radio-controlled model glider doesn't even have ailerons, and it has a flat-bottomed airfoil, yet it may be flown in sustained inverted flight.  (Roll control is challenging-- inverted, the ample dihedral is very destabilizing.)  With the right technique, nearly any aircraft would likely be controllable in sustained inverted flight.  Getting there safely from upright flight, and getting back safely to upright flight afterwards, may be another matter, especially if the roll rate is sluggish.
Obviously, whether or not the engines will receive adequate fuel and lubrication to keep running during sustained negative-G flight, and what will happen to the flight controls if they do not, will vary from one aircraft to another.

It can't be too hard in an MD-88. Denzel Washington did it while under the influence ;-)

This answer could be improved by adding a comment about adverse yaw as per https://aviation.stackexchange.com/questions/32865/during-inverted-banking-what-happens-to-heading-- I'll not do that right at this moment--  I see now that the "reversal" of adverse yaw has also been mentioned in another answer

Another aspect, not mentioned so far is that, because you effectively hold "bottom half" of the stick now (relative to the direction of aerodynamic forces), it is not only pitch but also roll control, which is in a sense inverted. But pedals and rudder stays the same, so in order to make coordinated turn you need to move the stick in opposite direction then the pedal you push on.
Or, in other words, you need to roll your head outside the turn, not inside, when flying inverted.
Depending on how your brain works when flying an airplane, this can be quite a difference for some.

Aileron and rudder remain "same side" coordinated no matter what orientation.  Inverted had my knees shaking at first, but gets much better with practice.  Forget about sky/ground reference, fly the plane where you want it to go.  Simulator or R/C experience will help build confidence.  Trust the physics (and trim that stick back to center).

Correct me if I am wrong (thinking right and left in various reference frames can get odd), but I believe the control is inverted in a sense. What I mean: if you want to turn right while inverted (let's say 'right' from pilot's perspective), you need to push with your right foot, but move the stick towards your left arm, right? Doing the right turn in non-inverted flight, you push right foot and move stick to the right. (I agree that it can feel completely normal and natural, if you think more what each control surface actually does, but the motion of your arm and legs is different.)

Martin, I wracked my brain a bit on this on too.  "same side" means rudder deflection is into turn, which must be delicate with negative wing loading.  Let's turn right inverted pilots perspective.  Plane W.R.T. ground turns left. Setting up the bank really confused me.  We would roll our heads away from the turn (left stick) and apply a bit more push on the stick and a bit more power to maintain lift.  The "inside" would be the right rudder pedal.  Our eyes, outside low, might have a tough time seeing what was ahead, but your inputs would work.

Most airplanes have asymmetric airfoils optimized for non-inverted flight.  Among other things, this means that stall characteristics could be much different (presumably worse) for an airplane in inverted flight.  I can't find it now, but I recently saw a video of two similar biplanes flying straight and level, one flying inverted directly above the other.  The upper one suddenly dropped, striking the other, and I believe both crashed.  Although I can't prove this to be the case (and I haven't heard the results of an official investigation), it seems plausible to me that the pilot of the upper airplane was unaware of the possibility of a significant increase in stall speed associated with inverted flight.

While you may be right that the increased stall speed contributed to the accident you mention, I find it hard to believe that stunt pilots would not have been painfully aware of that fact. Note: I presume they were stunt pilots well versed in their craft - most people don't put themselves in that kind of danger without knowing what they are doing (yes, I know there are idiots out there, I'm assuming these weren't among them).

I don't presume anything about their qualifications, and there was no way to judge from the video whether they were experienced stunt pilots.  Whatever their qualifications were, they were evidently insufficient to forestall a fatal -- and likely avoidable -- mishap.


text:
Which is safer? The Piper Tomahawk or the Cessna 152?
I have booked my first flying lesson for 2 weeks time, in my first step to obtain a PPL or LAPL. I had done all of my research on the Cessna family, assuming that is what I would learn in. I was satisfied by the safety and glowing reviews by students and instructors alike.
Anyway, my local school has since told me that they don't have the 152 anymore, so I will be learning in a Piper Tomahawk. Some quick reading here states:

The PA38 accident rate ranged from 0.336 to 0.751 fatal stall/spin accidents per 100,000 flight hours, compared to 0.098 to 0.134 for the 150/152.

and

There was always a concern about the stall and spin characteristics of the Tomahawk. The old adage “be careful what you wish for” applies because an overwhelming majority of the PA38 survey respondents requested an aircraft that would stall with more authority than the 152. And they got it.
You’ll hear many people talk about the stall characteristics of the PA38 like they’re referring to a demon, but it should be remembered that this was essentially a design feature to teach better pilots. Unfortunately, this led to a higher than average stall/spin accident rate that earned the PA38 a less than admirable reputation quite quickly (thus the nickname Traumahawk and Terrorhawk).

This isn't as comforting. So taking into account engine reliability, stall issues, etc. - which aircraft really is safer?

I should think whatever difference there is in stall characteristics pales in comparison to the effect of the pilot's handling.

What makes you think you will be doing a lot of stalling of the aircraft? (Especially solo.) And what on Earth does engine reliability have to do with stall handling? Trust me, pretty much the first thing any half-way decent instructor will be drilling into your head from the first lesson onwards is the emergency procedures checklist, front and center of which is establishing a safe airspeed well above stall speed, by gliding if you've lost engine power.

Consider: At 0.751 fatal accidents per 100,000 flight hours (the high end of the stated range), if you fly for 1,000 hours, your chances of being in a fatal accident are 0.751%. As I recall, you mentioned in another recent question of yours planning to fly for maybe one hour per month... At that point, your lack of currency might be a bigger problem than the aircraft's handling characteristics.

Thou shalt maintain thy flying speed lest the ground rise up and smite thee.

@Terry:  Airspeed is LIFE.

@DLH Altitude is insurance.

I did not downvote your question because your concerns are understandable, however they are misplaced. As a student pilot you are the greatest threat to your own safety, not the airplane. (That's what training is for)  I wish you good luck and success in earning your PPL!

I don't understand the downvotes.  It seems like a reasonable question from someone who doesn't yet know what variables feed into safety in aviation.  Also, it seems to me that someone who would ask this question is ahead of those who wouldn't think to ask it.

I'll admit I'm not particularly thrilled about this question myself, but really, primarily opinion-based? It's asking about something reasonably objectively quantifiable (accident statistics and the reasons for accidents) about two specific airplane model series. How is that primarily opinion-based?

From the Wikipedia article on the Tomahawk:

Safety record:
  According to the Aircraft Owners and Pilots Association Air Safety Foundation, which published a Safety Highlight report on the Piper Tomahawk, the Piper Tomahawk has a one-third lower accident rate per flying hour than the comparable Cessna 150/152 series of two-place benchmark trainers. The Tomahawk has a higher rate of fatal spin accidents per flying hour. The National Transportation Safety Board (NTSB) estimated that the Tomahawk's stall/spin accident rate was three to five times that of the Cessna 150/152.[2]

Earlier in the article it was stated:

Before designing the aircraft, Piper widely surveyed flight instructors for their input into the design. Instructors requested a more spinnable aircraft for training purposes, since other two-place trainers such as the Cessna 150 and 152 were designed to spontaneously fly out of a spin. The Tomahawk's NASA[1] GA(W)-1 Whitcomb airfoil addresses this requirement by making specific pilot input necessary in recovering from spins, thus allowing pilots to develop proficiency in dealing with spin recovery.

So what this means is that the Tomahawk was intentionally designed to be easy to spin so that instructors could teach students how to recover from a spin.  Other aircraft, like the Piper Cherokee, are almost impossible to spin so instructors cannot really teach spin recovery in those aircraft.
To answer your question on how this relates to safety that depends on how you look at it.  I would argue that a pilot who knows how to recover from a spin is a safer pilot than one who doesn't.  However since the Tomahawk is actually made to spin, there is the potential for increased spin/stall accidents if the pilot messes up.  Aircraft like the Cessna 152 or Piper Cherokee have design features that can "cover up" a pilots mistakes because they are difficult to spin or stall, but if you plan on flying aircraft that are not so hard to stall then not knowing the proper recovery procedures can get you killed.
The main factor in safety is not usually the safety of the plane but the safety of the pilot.  The vast majority of accidents are caused by pilot error.  I would worry more about evaluating yourself as a pilot and understanding if you have the right mindset to be a safe pilot.

They both crash if you fly them badly.  Stall/spin crashes generally happen while in the circuit, usually turning to final, and both airplanes will oblige if you mishandle them.  The 152's more passive spin recovery characteristics will do you absolutely no good at 500 ft AGL, so the discussion of spin recovery characteristics of the two airplanes is largely irrelevant.
Cloud, knowledge is power and all that, but be careful about taking some of this stuff to obsessive levels, which is going to work against you as you progress through this journey. You won't be able to enjoy yourself and will probably start to drive your instructor(s) bonkers.
Anyway, I would be more concerned with the condition of the school fleet and whether the school appears to maintain them well. Like, are the trainers kept clean, does everything work, are airplanes grounded a lot, is the maintenance shop a disorganized dump, that sort of thing.

Basically, as the flight school still has the plane flying the design is safe enough to be used for school usage. They do not wish to hurt either the student or the flight instructor. And they have done this quite a while: you are not their first student.
You will train in how to safely fly the plane under the guidance of an instructor. You will learn a lot about procedures and limitations and how to react in various situations. You can get into serious trouble in any airplane type — the instructor is there to teach you how to avoid it. Which of the generally available plane types is used is the last thing to worry about.


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What are techniques for balancing a radially folding propeller?
When balancing a radially folding propeller, such as the MagCAD 60x23, what are some techniques to employ?

A static balance is challenging because the orientation of the blades is not fixed.
Dynamic balancing might seem more natural, however vibration might also come from the way the blades pivot about their hinges as they swing through the propeller arc, similarly to a helicopter's lead-lag hinges.
And does it materially affect things if the blades are mounted horizontally or vertically? When they are mounted vertically, gravity will tend to advance the falling blade and retard the rising blade.

When those blades are spinning fast enough, it acts like a rigid propeller. I don't see the balancing procedure to be any different...

@mins, the prop in that video is an axially folding prop, so it cannot lead/lag. I don't know how that will affect vibration, maybe it doesn't at all?

@RonBeyer I don't expect that it's true that they act like a rigid propeller. The blades should constantly be changing position as they sweep through their arc, in function of gravity and whether the blade is operating in clean air or not. This might not matter for balancing and vibration, though.

Related, why lead/lag is rare on propellers per se: https://aviation.stackexchange.com/a/27356/31425

The folding of the propellers closely resembles ... a weed trimmer!
Given sufficient rpm, the blades will point straight out, with any flexing from twisting or bending helping to hold them in place on the hub. Once the blades are spinning fast enough, the effects of gravity are negligible.
One may balance the hub/prop by carefully matching the two halves in shape and weight, then temporarily locking them in the extended position.
Standard balancing with a gradual run-up of rpm to specs could then be safely performed for quality control.  Unmatching blades are discarded. The final, loose mounts are then installed and the prop/hub re-tested.


text:
EASA part 66 licence conversion to the Japanese version?
I am looking at a job proposal in Japan. I need to transfer my EASA Part 66 engineers license to the Japanese version.
Can anybody offer any advice on how I would achieve this?

You'll probably need a letter from your part 66 licence holder's authority.
I know people who get equivalent between French and Australian.
The best approach is to contact the Japan's competent authority (JCAB-MLIT) for the application details.
Regards


text:
How is airline fuel hedging actually done?
When airlines refuel at the airport are they all billed at the same rate on the day? Similar to cars at a gas station?
For example if Southwest has hedged and paying a dollar less per gallon than its competitors, how is that dollar realized? Are they billed at a different rate? Or is fuel hedging purely a stock market contract game where the profits or losses are just added/subtracted to the actual fuel costs after the fact at the accountancy/financial results reporting stage?
It’s usually described that airlines hedge against WTI or similar but as they are actually buying kerosene I assume the hedging must be an indirect insurance vs a literal contract to the supplier of the fuel?
Many questions but all essentially ask how does fuel hedging work in practice? I understand the basic concept of buying options at a particular price and selling above below the spot price at a future date but how does that actually work for airlines in practice at the fuel pump?

I am far from an expert on this subject, but I'd imagine it works something like this:

Airline A contracts with FBO B (or the airport) to purchase 50,000Kg of fuel per day for \$1.99 per Kg.

N12345, owned by Airline A, pulls up to gate A27, FBO B dispatches a truck to the gate and pumps 10,000Kg into the plane, making a note of the date/time/gate #, how many Kg were pumped, and the tail number of the plane in question.

This is repeated several times per day for the various aircraft of Airline A that need to be refueled.

At the end of the month, FBO B sends a bill to Airline A for \$2,985,000 for 50,000Kg of fuel at \$1.99/Kg per day times 30 days, along with an itemized invoice showing how much fuel was actually pumped into which aircraft.

If Airline A actually used less that 50,000Kg of fuel for a day, they probably still have to pay for it per the contract.
If Airline A used more than 50,000Kg of fuel on a given day, there is an extra charge, at a pre-agreed price (maybe the same \$1.99, maybe \$2.50, who knows - it depends on the contract) for each additional Kg of fuel pumped.

Airline C may have negotiated a contract with FBO B for fuel at the same airport and agreed to pay \$2.15 per Kg for their fuel.

It could be the same 50,000Kg/day as Airline A, but they negotiated at a different time when it appeared that prices would be higher in the future.
It could be that they negotiated for a smaller quantity of fuel, so have to pay a higher price for a lower usage.
It could be that their negotiators just aren't as good as those at Airline A, and that they need to go poach some better people for their purchasing department.

At the end of the month, FBO B sends an invoice to Airline C for \$3,225,000 (assuming the same 50,000Kg/month) for their fuel with adjustments for any overage.

*Note: all names/prices/quantities/tail numbers/etc. are purely made up for this example and have little to no bearing on reality.

Excellent answer, but @federico, is speculation about accounting practices on topic?

Thanks, @MichaelHall. I'd agree that this may just be a general accounting/finance question. It could go to [money.se], but it's not exactly Personal Finance. It might be on topic at [quant.se], but I don't understand enough about finance to know one way or the other on that. It's definitely not [travel.se]! :)

And personally I'd like to see it stand, my comment stems from another thread that in my opinion was worth keeping open.  I know people aren't always going to agree, but sometimes the enforcement seems a bit arbitrary.  Sorry to use this as an "example", I will delete my comments at an appropriate time...

No worries! I've been known to jump to "migrate it!!" a little too often. ;)

@FreeMan it's definitely not quant.

@hobbs - my understanding of finance ends when my checkbook balances and I know I've spent less than I've made in any given month. I'll take your word on it. ;)


text:
Why aren’t multiengine general-aviation aircraft required to be capable of sustained single-engine flight?
While perusing the See How it Flies website, I came across this alarming statement regarding multiengine general-aviation aircraft:

You must not allow yourself to think that just because airliners can climb with an engine out, your favorite light twin can climb with an engine out.
It is legal to operate a light twin with anemic or nonexistent single-engine climb performance. In such cases, engine failure at low altitude is perhaps the most critical situation that arises in general aviation with any appreciable frequency. Like a single-engine aircraft with partial power failure, you need to make a forced landing.

Given that the main point of having multiple engines is the ability to maintain flight if one of them kicks the bucket in midair, why are GA aircraft with more than one engine allowed to be incapable of maintaining single-engine flight when all this does is double (or triple, or quadruple) the likelihood of a forced landing subsequent to an engine failure?

There is an old saying for light twins: "the second engine is there to get you to the accident site faster" (or variations of it)...

Note that the first paragraph refers to climbing on one engine; many (most?) GA twins can at least maintain cruise altitude on one engine, so takeoff is the main problem. Even if they can't, half power gets you a lot more range to pick a good forced landing site (maybe even an airport) than zero power would.

If someone has an answer, could they consider in their answer the statistical possibilities of one out of two engines failing, three out of four engines failing with modern GA aircraft. That might be too broad of a response. I would think with engines from the last twenty years, reliability and predictability of failure has increased the Mean Time Between Failure.

I'm not sure what that site is, part blog, part other stuff, I wouldn't consider it authoritative.

@GdD - I consider it one of the best flight-dynamics-for-pilots resources I've ever encountered.  I have yet to find any errors in it.

It is legal to operate a light twin with anemic or nonexistent
single-engine climb performance.

No, and technically it would not be certifiable as airworthy as per § 23.67 Climb: One engine inoperative. Aircraft that were certified at a different point in history may subscribe to different regulations.

(a) For normal, utility, and acrobatic category reciprocating engine-powered airplanes of 6,000 pounds or less maximum weight, the following apply:
(1) Except for those airplanes that meet the requirements prescribed in § 23.562(d), each airplane with a VSO of more than 61 knots must be able to maintain a steady climb gradient of at least 1.5 percent at a pressure altitude of 5,000 feet with the -
(i) Critical engine inoperative and its propeller in the minimum drag position;
(ii) Remaining engine(s) at not more than maximum continuous power;
(iii) Landing gear retracted;
(iv) Wing flaps retracted; and
(v) Climb speed not less than 1.2 VS1.

In other words if you can get to ~5000 Ft. you need to be able to maintain a nominal climb in a light twin.
The regulation for aircraft over 6000 lbs. is similar as well depending on where you draw the line for "light". There is also more information on how the testing occurs in this AC.

What if you're taking off in your light twin from LAX (or other airport at/close to MSL) and are still well below a 5,000' pressure altitude when one engine goes pop? After all, 5000 feet gives you a few minutes of glide time to get yourself organized. 500 feet, not so much.

@FreeMan I was thinking the same thing. Strangely it only seems to be covered by the regs for aircraft over 6000 LBS where your single engine climb out needs to match your 50ft climb out as per sub section (f)

@FreeMan Would performance not be better at MSL than at 5000ft?

A very important bit in that regulation is that it applies to aircraft with $V_{s0}$ more than 61 knots. Well, guess what, many twins out there have $V_{s0}$ (flaps down stall speed) of exactly 61 knots, so the regulation does not apply to them. Which suggests the answer should, actually, be yes.

I remember reading in a popular aviation magazine years ago that statistically, you were more likely to perish in a twin-engined aircraft with an engine out than you were in a single-engine aircraft with an engine out. It all centered around lack of proficiency in singe-engine ops in a twin, mostly near the ground.

@RetiredATC, I would believe a statistic like that, even if it initially might seem counterintuitive.  However, once I watched a cub reporter on TV exclaim insightfully and without the least hint of irony or mathematical awareness, "statistically you are twice as likely to experience an engine failure in a twin engined aircraft..."

Given that the main point of having multiple engines is the ability to maintain flight if one of them kicks the bucket in midair,

I think that assumption is flawed. Generally, twin engines are there to be able to haul more payload into the air, not to be resilient to engine failures.

To clear up confusion; most light twins do not climb on one engine were certified as such:
FAR 23.67 provides the single‐engine climb performance requirements to airplane manufacturers for FAA certification of multi‐engine aircraft.

For aircraft with a maximum weight of 6,000 lbs., or less and a VSO of 61 knots or less: The single‐engine rate of climb at 5,000’ MSL must simply be determined with the—

Critical engine inoperative and its propeller in the minimum drag position
Remaining engine(s) at no more than maximum continuous power
Landing gear retracted
Wing flaps retracted
Climb speed not less than 1.2VS1

• The rate of climb could be a negative number – meaning a descent
• There is no requirement for a single‐engine positive rate of climb at 5,000 ft., or any other altitude.
For Aircraft with a maximum weight of 6,000 lbs. or less, and/or VSO more than 61 knots: If certified before February 4, 1991: the single engine rate of climb in feet per minute at 5,000’ MSL must be equal to at least .027 VSO2.
If certified after February 4, 1991: maintain a steady climb gradient of at least 1.5 percent at a pressure altitude of 5,000 ft. with the —

Critical engine inoperative and its propeller in the minimum drag position
Remaining engine(s) at no more than maximum continuous power
Landing gear retracted
Wing flaps retracted
Climb speed not less than 1.2 VS1

NOTE
Do not confuse the date of type certification with the airplane’s model year.

OP’s question is “why’  This doesn’t seem to answer that.

FAR 23.67 was already quoted here. Also, using a > formats text to indicate that it's a quote and makes it much easier for others to read.


text:
Why do aircraft stall warning systems use angle-of-attack vanes rather than detecting airflow separation directly?
Stall warning and protection systems on essentially all aircraft work by using several hinged vanes on the side of the fuselage (usually at least three, and preferably more), which align themselves with the airflow over the aircraft’s surface, and, thus, provide a measurement of the aircraft’s current angle of attack.  The aircraft’s computers then compare the aircraft’s angle of attack to a preloaded value slightly below the aircraft’s normal stall angle; if the angle of attack is lower than this threshold value, nothing happens, while, if the angle of attack is greater than the threshold, the computers sound the aural stall warning and activate the stickshaker.1
These systems, although mechanically simple, have the major disadvantage that they do not actually detect an impending or actual stall itself; instead, they detect when the aircraft is approaching its usual stall angle, and assume that the usual stall angle also happens to be the current stall angle.  Which is all well and good, if your aircraft has invulnerable wings devoid of high-lift devices which are magically immune to things like ice and insect accretion, but most aircraft have wings that

have high-lift devices on the leading edges, the trailing edges, or both (leading-edge devices, such as slats and droops, dramatically increase the wing’s stall angle, while flaps, mounted on the trailing edge, slightly decrease the wing’s stall angle);
can gather considerable quantities of ice (in the cold season) or insects (in the warm season) under the wrong conditions (both ice and insects increase the roughness of the wing’s leading edge [and, sometimes, the entire upper surface], dramatically decreasing the wing’s stall angle); and
are possible to damage, including on the leading edge and upper surface (damage to the surface of the wing, especially on the leading edge, increases the wing’s roughness, dramatically decreasing the wing’s stall angle in the same way as ice or insect accretion do).

Stall-warning systems on newer aircraft try to take this into account, such that an aircraft’s stall warning threshold might be set to (say) 25º when clean, 45º with slats extended, and 5º if icing is detected.  However, although the aircraft’s clean stall angle is easily measured during flight testing, as is the stall angle with various high-lift-device configurations, the stall angle of an iced, insected, or damaged wing cannot meaningfully be determined in this manner, as it depends on the precise amount, shape, texture, etc., of the ice/insects/damage, which have a near-infinite number of possible configurations; as such, the threshold in icing conditions is mostly a guess, and could easily be far lower, or far higher, than the actual stall angle of a particular icy wing, and most stall-warning systems don’t even try to account for insect smears or wing damage.  To make matters worse, icing, insects, and damage are seldom symmetrical between the two wings, such that one wing could easily end up with a considerably higher stall angle than the other; an angle-based stall-warning system will provide no indication of this.
It is, however, possible to detect an impending or actual stall directly.  As an aircraft’s attack angle approaches whatever its stall angle happens to be at that particular moment, the airflow over the upper surface of the wing starts to separate from the wing; this flow separation begins at the trailing edge, and the region of separated flow extends forwards as the aircraft comes closer and closer to stalling, until, when the aircraft finally stalls, the flow separation covers nearly the entire upper surface of the wing.  This flow separation, and the associated powerful turbulence, can easily be detected by a pressure and/or sound sensor on top of the wing, and this technology was first available over a quarter-century ago; to quote the NTSB:

There is new technology available that can detect airflow separation on aerodynamic surfaces. One new system measures the pressure in the airflow above the upper wing surface with a probe located at about 70 percent chord (varies by airplane), inboard of the ailerons. The system has been shown to effectively detect upper wing surface turbulence associated with airflow separation, both in flight and during the takeoff roll, once the airplane has accelerated to at least 50 knots. According to a Society of Automotive Engineers (SAE) technical paper,77 developmental testing found that:

Conventional stall warning systems, which use a fuselage mounted AOA sensor, do not measure the actual stalling condition at the wing. The key to determining an early stall due to the presence of contamination is to measure the flow directly at the lifting surface. Local velocity changes in a region above the upper surface of the wing provide a consistent indication of an approaching aerodynamic stall even when contamination is present. This method of stall warning also offers new levels of safety during low level windshear recovery and takeoff performance monitoring.

ATR had established a test program that evaluated the effectiveness of this device before this accident. The system was tested on an ATR 72 in January 1994; and it was tested on ATR-42-500 in December 1994. In addition to ATR airplanes, tests have been conducted with the detection device mounted on a Cessna 421 (cabin-class piston twin), a NASA Sabreliner (business jet), and a Fokker 100 (passenger jet). Additionally, wind tunnel tests were conducted with various amounts of surface roughness and ice shapes on various airfoil designs.
A second new type of airflow separation detection system measures the change in sound (amplitude and frequency) of the airflow over the surface of an airfoil. This system had not been flight tested before this accident, but subsequent wind tunnel tests at the NASA Lewis Research Center showed consistent reliability in the detection of airflow separation.
77SAE Technical Paper 922010, Stall Warning Using Contamination Detection Aerodynamics, by Paul Catlin, B.F. Goodrich Aerospace Avionics Systems, Presented at Aerotech '92, October 1992.

(Pages 156-157 [paper report]/174-175 [PDF report].)
Yet, even though systems that detect a stall or impending stall directly have been available for decades, and such systems are able to reliably detect a stall no matter what the aircraft’s stall angle is2 (which a vane-based system could never even come close to doing), the overwhelming majority of aircraft (even the large airliners where a stall would be the most-potentially-catastrophic) still use attack-angle vanes as the basis of their stall-detection and -warning systems, instead of using systems based on airflow-separation detectors.
Why?

1: T-tailed aircraft have a stickpusher as well as a stickshaker, due to the risk of the aircraft entering a difficult-to-recover-from deep stall if the stall warning is ignored; the stickpusher activates at a somewhat higher attack angle than the aural warning and the stickshaker.
2: Or, for that matter, if the aircraft doesn’t know what angle of attack it’s flying at (and, thus, can’t even begin to determine where it lies relative to the threshold) - for instance, because it doesn’t know how fast it’s going.

There are various systems on general-aviation aircraft that use the movement of the stagnation point / line on the leading edge of the wing to trigger stall warnings. https://bondline.org/wiki/Stall_Warning_System

You might consider adding the airliner tag here; saying that "essentially all aircraft" have AoA vanes and air data computers isn't accurate when you consider light GA aircraft.

@Pondlife: Done, added.

The short answer is that we want to warn the pilots about an impending stall well prior to an actual stall condition.  From a safety perspective, waiting until the airflow starts to separate, or at the onset of buffeting, it's already too late.
By using AOA we can set a very conservative threshold.  This AOA threshold can be adjusted by taking into account many factors such as the flaps/slats configuration, airspeed (mach number), etc.
By nature, sensors which work by detecting airflow separation cannot be used to provide this early warning.

Nice answer, and welcome to Av.SE!

If you ever fly with tufts on a wing, you’ll see that the airflow can get disturbed on wing sections well before full-on wing stall. So I think this sort of sensor would in fact give early warning if positioned correctly.

In looking into my comment about stall warning devices, I found, in the US airworthiness standards for transport category airplanes, 14 CFR § 25.207, which states specifically how much before the stall the warning should commence: https://www.law.cornell.edu/cfr/text/14/25.207

@MikeY Good point, and in airplanes that display pre-stall buffet, that's what is causing it. The standards I quoted above say an acceptable warning may come from "the inherent aerodynamic qualities of the airplane." They don't explicitly say buffet is one such quality, but if it is, then the detection of pre-stall separation would seem to be a candidate basis for a warning device, though it must work in all configurations and not be prone to false positives.

@MikeY Unlike on straight wings, on many swept-wings the airflow separation may start at the wing-tips, very close to the control-surfaces, so there is little margin between the onset of stall and some loss of control effectiveness. Also, on swept-wings the stall boundary is much less abrupt than on straight-wings, making stall detection via flow separation more difficult. Incidentally, this is a reason why we need stick shakers to begin with -- on many swept-wing jets there may be conditions with little (or no) natural pre-stall warnings, so an artificial warning had to be introduced.

@sdenham Just to note that §25.207 does not precisely define the stall warning speed (Vsw), but only specifies the minimum margin between the stall warning speed and the stall speed. That is, Vsw must be at least [the stall speed + required margin]. Again this is a conservative requirement that the stall warning should be activated well prior to an actual stall, as clearly stated in §25.207 (a).

"The short answer is that we want to warn the pilots about an impending stall well prior to an actual stall condition. From a safety perspective, waiting until the airflow starts to separate, or at the onset of buffeting, it's already too late."  The onset of flow separation is well prior to the actual stall (for that matter, small areas of flow separation often exist at and just forward of the trailing edge of parts of the wing even in normal flight!).

Yeah, I get that its good to get plenty of warning, but as the OP points out, there are ways for that to fail, so why don't we have anything to warn when an actual stall is imminent/occurring?

To expand on peekay's point, the bottom line is that waiting for actual flow separation to occur is waiting way too late for a stall warning system, except on aircraft with extremely mild and benign stall behaviour.  For aircraft that do have very benign stall behaviour, you already have a flow separation detection system, known as pre-stall buffet (You can take this to an extreme in a 1-26 glider, which has a nicely progressive stall with a powerful shuddering buffet at its onset, just below minimum sink speed.  You thermal a 1-26 just on the edge of the buffet, shuddering and rumbling your way around).
But for lots of aircraft you just don't want to get to or close to stalling AOA in the first place (aircraft with stick pushers have them because the stall itself is potentially fatal, and the pusher is a last chance at avoiding it) and relying on detection of flow separation is of no use.  You have to have margin, and that means you have to have AOA.
And not just AOA on complex aircraft.  Stall protection computers use the AOA as one of several data inputs.  Aircraft deceleration rates and other factors are all included in computing a solution that sets the top of the barber pole, and sets shaker and pusher (if installed) firing points with adequate margin for a particular flight condition.

Can't they slightly distort a small section of wing so it displays detectable behavior earlier than the rest?

Airplanes with good stall behaviour do exactly that with various means, like rectangular planforms, stall strips to trip flow early, or wing twist (washout) that makes the tips always see a lower AOA than the root, .  They all make the stall start at the root and progress outboard.  The turbulence from the separation at the root makes the tail shake.  A lot of high performance a/c though, have wings that let go all at once and those techniques aren't really feasible for performance reasons, so you end up with shakers and in some cases pushers.

There are really good reasons to have AOA gauges other than for stall warning, so they are already there. Probably an element of wanting to keep the systems simple?
For the majority of operations the AOA indicators are great. For cases like icing, hopefully you know there is icing, and you are flying with margin. With an iced wing, stall warning can be too late. Still, I’d not complain if I had a true stall sensor.
If the FAA required them, then we’d see them. So probably a big policy element in all of this as much as engineering element.

Except AOA is not simple.  AOA sensors are easy enough, but deriving stall from their indication requires a whole lot of assumptions, including feedback (are the slats actually in the position commanded?), and data about the aircraft (weight and balance) you have to trust the pilots on having entered correctly, like fuel.


text:
Why do the 737-100/200’s thrust reversers blow fully open if hydraulic pressure is removed while the reversers are partly open?
If the thrust reversers on an Original-series 737 (the 737-100 and -200) are 2 inches or more from the stowed position, and hydraulic pressure is removed from the reverser actuators, the reversers will travel to the fully-extended position under aerodynamic loads at airspeeds down to below 130 KEAS (knots equivalent airspeed), as was discovered during the investigation into the crash of Pacific Western Airlines Flight 314.  To quote the official accident report:

... It was found that the air loads on the reversers were insufficient to deploy them further if they were freed with the leading edges 1 inch out from the closed position. If the initial deflection was increased to 2 inches, they invariably deployed fully under the air loads. At intermediate settings, deployments only occasionally occurred. ...  [p. 17-18 of the report; p. 23-24 of its PDF file]
... The ground runs showed that partially opened thrust reversers deployed at various speeds during taxi tests with manually controlled airlground logic. The rate of deployment varied with the amount of the initial open condition of the reverser doors and also varied with speed. In all cases partially opened doors deployed fully before 130 Kts.  [p. 18/24]

However, the 737 Originals use target-type reversers (you know, the kind with the two buckets that close behind the engine’s tailpipe, the ones that everyone immediately thinks of when they hear the phrase “thrust reverser”); I can easily see how they would be blown to the full-open position if the engine(s) aren’t operating, but, if they are running, then, without hydraulic pressure holding the reverser buckets in position, shouldn’t the engine(s)’ jet blast force the buckets apart and open a path between them for the engine efflux to exit?

If the buckets could be opened by engine thrust, they would be ineffective to reduce speed. By the same token, you want airflow to help them deploy when needed. The controlling force then is the hydraulic actuators. Without hydraulic power it would be very difficult to achieve positive control over the buckets' position.

If the buckets could be opened by engine thrust, they would still be effective at reducing speed as long as their hydraulic actuators could hold them closed against the force of the engine's airflow.

@Sean: But if you're landing and using thrust reversers, I'd think the design assumption would be that you need them to keep working in order to stop before you run off the end of the runway, no?

@Sean That doesn't make any sense. If the hydraulic actuators can hold them closed, the buckets cannot be opened. If they can be opened, the actuators are useless and would be ineffective in slowing the aircraft.

@JuanJimenez: If the hydraulic actuators can hold them closed, the buckets cannot blow open as long as the actuators continue to receive hydraulic pressure - they would still be useful for braking, but could potentially blow open if hydraulic pressure to the actuators was lost.

No @Sean they won't be, because no redirection of thrust happens until the buckets open.

@JuanJimenez: No, the buckets redirect thrust when they shut behind the engine - they stop redirecting thrust if they reopen.

@Sean you are confusing the buckets with the redirected exhaust and actuation. At any rate, there's no point arguing this, the end result is the same, it's just terminology.

Fortunately the airflow around the engine at that phase of flight is hopefully faster than the airflow passing inside the engine, since you're in need to slow down the plane. In every other phase where you need engine thrust it would probably interfere as you asked. As well as for petal reverser, airflow keeps it wide open.

If the flow inside the engine is slower than the freestream, then it's producing drag, not thrust. Even at idle jet engines typically provide some amount of thrust.


text:
Can a 777X legally be ferried with one or both wingtips folded?
The 777X family (777-8/-9/-10) features wingtips that fold up on the ground to let the aircraft fit in tight spaces. These have to be extended and locked in place before a revenue flight can take place, but can a 777X legally be ferried with one or both wingtips in the folded position - for instance, if one of the wingtips is jammed in the up position while at some backcountry airport that doesn't have maintenance facilities for a 777?
For the purposes of this question, I'm assuming that it's physically possible to operate the aircraft. The non-folding part is the same length as the wing of a regular 777, so it would still be able to take off safely, just with considerable performance penalties compared to normal, wingtips-folded-down flight. Ferry flights for maintenance purposes can get away with a lot that wouldn't be allowed during line operations (for instance, multi-engine jetliners can be ferried with one engine - or, for quadjets, two - inoperative, and the 747-400 can be ferried with one of its winglets broken off).

Having one wingtip folded and the other unfolded would result in asymmetric lift. I'm guessing that this would result in a fatal loss of roll control, even with the aircraft empty. There is also the risk that a folded wingtip would break off as a result of aerodynamic forces in flight.

@Thomas I'm not sure that a missing 3.5 meters wingtip will cause a fatal loss of roll control

What "backcountry airport" is going to be receiving 777s?

@DavidRicherby a diversion one

If the aircraft managed to land at an airport without a maintenance facility, they would fly in the engineers/equipment to repair it before it took off again. This isn't unheard of in the industry. So no, they would not take off knowingly having an issue with the folding wing tips.

@DeepSpace a 3.5m wingtip at the end of a 32m lever (a.k.a. Wing) creates a lot of torque.

@Thomas The lift drops off significantly as you go towards the tips. The integral under the curve of the last 10% is small.

Are you asking if it's physically possible, legally possible, or both?

@Pondlife: Legally possible - it's certainly possible physically.

Thanks for clarifying, are you asking about regulations in the US?

@Pondlife: I suppose, yes.

“for instance, multi-engine jetliners can be ferried with one engine - or, for quadjets, two - inoperative” - No, you cannot ferry these aircraft like that.  Loss of an engine is considered an emergency situation.  The fact that is can still fly is not grounds to permit ferrying the aircraft on.

As Ron notes in the comments to another answer this is a perfect situation for a Ferry Permit/Special Flight Permit. The FAA via your local FSDO can authorize aircraft to fly for the purpose of being fixed elsewhere via a ferry permit. This question covers whats needed for that.
However in some cases it may be cheaper to fly (or drive) in a maintenance team and fix it on site. Moving an empty 777 even short distances means burning quite a bit of Jet-A. For the cost of the reposition it may just be cheaper to send a team (possibly in car) with the parts and tools they need to get the job done.
On a similar note its likely that the majority of fields big enough for a 777 to touch down have at least some form of maintenance on hand. Again it may be cheaper for an airline to contract the job to a local shop than bother with the ferry logistics and costs.
Boeing also offers these services if you should need them in a pinch as part of the AOG services.

Our technical experts will provide on-site, comprehensive and
  integrated assistance to recover an airplane. Our goal is to quickly
  and safely return airplanes to service, using methods and procedures
  that avoid costly secondary damage.

From what I can tell they will basically show up with parts and people to get the plane back in the air just about anywhere you can fly it out of.

If it's just flying mechanics on site with personal tools and some parts to swap out parts or perform an MMEL M procedure, that's one thing, but if a procedure requires tooling or special equipment that isn't available on site and has to be shipped as cargo, the logistics become extremely expensive and time consuming.  An airline will almost always try to ferry an airplane to a maintenance base where it has the equipment if it can.

@JohnK KLM regularly flew crews and equipment, including rigs and spare engines, all over the world to fix their MD11s which were kinda prone to problems with the center engine. Was apparently cheaper than flying out an empty aircraft to bring home the pax, and then ferry the MD with its faulty engine back home for repairs (if it could even get out on 2 engines of course).

Not with one tip up and one tip down, but it could be possible to ferry with BOTH tips folded, or removed, if Boeing has tested it.   A scenario might go like this:

Airline can't get one wing tip to unfold, or the tip gets physically damaged somehow, at a location where required equipment is unavailable.
Airline contacts Boeing customer support (why not; they have nothing to lose by asking) and asks if they can get an authorization for a non-revenue ferry flight with both tips folded or removed.
There will be an internal discussion within the Boeing engineering/test/customer support organizations.  It'll depend on whether Boeing did any experimental flight testing to document the airplane's behavior with tips folded or removed.  If not, it will probably end there and the airline will have to fly facilities to the airplane instead of the opposite.
If they have test documentation that demonstrates that airplane can be flown safely with the tips folded or removed, and the engineering heads who have to sign off have done a risk evaluation and are comfortable (and their relevant FAA counterparts are ok with it), it's theoretically possible that the FAA could issue a one-time ferry permit with a list of operating limitations (such as weight, altitude and speed restrictions).  My guess would be a flight with tips removed is the most likely configuration to be authorized, if at all.

I'd be surprised if this sort of thing hasn't already been evaluated within Boeing and has been determined to be feasible or not feasible.  Only someone within the Boeing engineering organization would know.

Boeing can't issue ferry permits, only the FSDO can, which they would probably want Boeing's blessing.

Ok I modified my post.  I was with an OEM in Canada and we would issue ferry permits by delegated authority but for something like this they would probably want Transport Canada's approval as well, so same thing in practical terms.  In any case, FAA certainly wouldn't go near it if Boeing hadn't done some testing and was confident there were no issues.

@Ron Beyer: Perhaps we're using a different definition of the word "can"?  I take it to mean the ability of the plane to actually fly, not the legality of the FAA or whatever body issuing a permit.

No doubt it would fly even with one up and one down.  The asymmetry in wing area is not that great.  You could easily offset it with some fuel imbalance.  With them both up or removed it would be almost unnoticeable, a negligible reduction in total wing area, notwithstanding vibrations/flutter or other side effects from exposed bits.  You never know what hidden side effect can jump out of the shadow.  It would have to have some minimum level of test, or at least very convincing and complete engineering calculations and analysis.

@JohnK: If you're worried about flutter, fly slower.

Flutter can happen at any speed; down around the stall if conditions are right/

@JohnK: ...but the conditions are far more likely to be right at higher speeds.

So what?  I don't need to test then?

@JohnK: No, just ask Boeing for the flight-test data they already have.

Which is what I said in my answer point 4.

I don’t believe Boeing authorizes flight of the 777x with a wing fold failure.  To be blunt, if you can afford a \$425 million airplane, you can afford a \$30,000-\$40,000 maintenance call where an AOG team is dispatched to the airport to repair the airplane and return it to service.  Even if Boeing approved flying the airplane with a wing fold failure, I doubt your Company SOPs, not to mention the insurance underwriters would approve.

It’s often not money but time to fix which is the issue. Flying to next base might be more expensive in fuel but save days or weeks in parking and lost revenue.

Well that’s why they dispatch an AOG team like that.  They can usually get the problem resolved in a day or two.  Again both the airlines will prefer that cost to a hull loss in a crash and subsequent lawsuits and bad publicity.

It depends on the issue. In my airline, we use whatever means comes out best. All I‘m saying is that sending an AOG team is only one tool in the box and that a ferry flight might be the preferred option. After all, three engine ferries on quads or gear down ferries are, while special events, not unlikely to be at least considered.


text:
What's this turquoise stuff covering the overlap between the blast pads of KBOS runways 04L and 09?
Although runways 04L/22R and 09/27 at KBOS stop just short of intersecting, the blast pads for runways 04L and 09, which continue some distance beyond the ends of the runways proper, do intersect.  The portions of said blast pads south and west of where they first touch are entirely (except for the area supporting the runway-27 localiser array) covered with some sort of turquoise substance (according to Google Maps, at least):

What is... whatever that is, and why is it there?
For comparison, here're those same blast pads back in 2018, when they had just the normal yellow chevrons:

Not a dupe of this question:

The green-marked areas there are in entirely different types of locations compared to the ones here (those markings are along the sides of runways and taxiways; the ones here are on blast pads).
The markings there are the wrong shade of green.
If the explanation here (coloured stuff marking a no-drive/no-taxi area) were the same as there, one would expect there to be turquoise stuff in a lot more places than just that one particular set of blast pads (not to mention that, as explained in @ymb1's comment, blast pads are already no-go areas except when they're being used as stopway during a runway overrun, making extra colourant redundant).
The weirdly-coloured stuff in this question, unlike that in the other question, actually obliterates a set of markings that are generally highly important (the blast-pad chevrons on the affected areas), thus tending to require a stronger justification for its presence than otherwise.

Does this answer your question? What are the areas marked in green around the taxiways?

@RalphJ: No, it doesn't, given that a) that question's green-marked areas are in entirely different types of locations, b) they're the wrong shade of green, c) if the explanation here were the same as there, one would expect there to be turquoise stuff in a lot more places than just that one particular set of blast pads, and d) the weirdly-coloured stuff in this question, unlike that in the other question, actually obliterates a set of markings that are generally highly important (the blast-pad chevrons on the affected areas), thus tending to require a stronger justification for its presence.

Did someone do some green-screen filming there?

Maybe it’s the last unfinished work of late artist Christo ;)

My first thought was if perhaps they were an EMAS installation; the new thing is "green EMAS" which is not literally green (I don't think) but is made of recycled glass and other materials. But the airport diagram doesn't show EMAS in that location.

Have voted to leave the question open, does not seem to be a dupe.

Can anyone confirm that the colouration is indeed real, not just some artifact of Google's photo? I'm curious about the cut-out section either way.

To answer my own comment, seems to be real, and please compare with a patch a little to the north, just off the end of 15R

I see three other turquoise areas in that airport.  None of them are blast pads.

KBOS airport, seen from East, source: Jet Photos
Short answer
In 2015, the airport owner started a project to accommodate for larger category VI aircraft (Airbus A380 and Boeing 747-8). The project consisted in Terminal E modification and runways and taxiways local pavement reinforcement.
For the project to be accepted by authorities, it had to satisfy multiple environmental criteria. The chosen solution included offsetting pavement additions by removal of an equivalent area of existing pavement. This removed portion included a part of runways 9 and 4L blast areas. Removal details:

To offset new areas of pavement, Massport proposes to remove pavement on the airfield. These areas would be planted with grass to offset grassland lost through the proposed airfield improvements.

Reinforced areas are depicted in blue below, removed areas in orange:

The overlapping blast area between the two runways was ultimately decommissioned, though, as visible on Google Streetview when looking carefully, the pavement wasn't removed, and there is no grass yet.
Instead the area was painted in this green-blue color, which is the recent standard used for "NO TAXI" islands. Smaller patches of this color can be found at other places on the airport.
Runway 4L and 9 thresholds are not located at the beginning of the paved strips. This is for approach path reasons. But the unused chevron-covered portions are not part of the runway safety areas (RSA) either, in particular they are not published as stopways.
Details on markings and RSA follow.

When is green paint used on airports?
Green paint or artificial turf can be used for airport markings for areas not able to sustain the load of an aircraft. Such color is used for innermost borders of taxiway shoulders. The inner area may also be filled with green color (AC 150/5340-1 Standards for Airport Markings):

The continuous taxiway edge marking consists of dual, continuous lines (2) with each line being at least 6 inches (15 cm) in width [...] Although it is preferable for the inner portion of NO-TAXI islands to be unpaved, for example, grass covered, the inner area may be painted green or painted with striated yellow markings per paragraph 1.3.d.

An example at KBOS:

What sort of areas can extend the runway?
Runways can be extended by:

A blast pad used as a protection against deterioration and projections by jet blast. A blast pad is not guaranteed to sustain aircraft weight. It is not a stopway, and it cannot be used in landing or takeoff calculation.

A stopway used as a last resort way to decelerate aircraft in a landing or a rejected takeoff. A stopway is able to receive aircraft without damages.
A stopway is declared (AC 150/5300-13 §322) and its size appearing in the airport diagram and FAA A/FD can be used in calculations.

A EMAS is made of specific frangible materials to completely stop the aircraft with little damages to the gears. Such systems are found on 22R and 33L runway ends (visible above in white), and one is planned for runway 27 end. An EMAS is not a stopway.

A clearway to create a free corridor for departing aircraft.

All these areas are covered with yellow chevrons, except for clearways over water, so landing aircraft are reminded they are not part of the runway. Stopways and clearways can be included in takeoff performance computations. A clearway increases TODA and a stopway increases ASDA:

TORA, TODA, ASDA, LDA
What are they in the case shown?
On the airport diagram (source):

There are no size indication in the airport diagram and when looking at TODA and ASDA distances in A/FD, we see TODA and ASDA are the same than TORA:

RWY 22R:TORA–7864 TODA–7864 ASDA–7864 LDA–7046
RWY 27: TORA–7001 TODA–7001 ASDA–7001 LDA–7001

Therefore the areas are neither stopways nor clearways. They are blast pads. From FAA regulation standpoint, a blast pad is not part of the runway safety area (RSA), and its removal doesn't change the runway declared lengths.

So what is the green area made of? If I read your answer right it should be grass, due to the removal of the blast pad pavement to allow for the addition of pavement elsewhere on the field. Is it painted pavement or painted grass?

How would one be able to have a blast pad that can't support the weight of an aircraft, given that the blast pad has to be right up against the end of the runway - i.e., forming the first part of the mandatory runway safety area, which does have to be able to support an aircraft's weight?

@randomhead: I think the environmental requirement for removing the pavement is to maintain the same level of water draining (from what I read in the project). When you look at the picture you clearly see the chevrons behind what seems to be paint. Did they remove the pavement? I can't say for sure, but it seems the pavement is still there.

@Vikki: To me a blast area is not a stopway. There are no published stopways on these ends of the runways. Stopways are indicated by a different ASDA, and by mentioning  their size on the airport diagram. More.

I think I see what you mean: That whole area is still covered in pavement, but they ripped it out and covered that same area with less volume of asphalt (for better drainage), so it can't support an aircraft anymore. And then they painted it green to make it clear that it couldn't be taxied on. Am I on the right track now?

Hm. The document clearly says that the proposed removed pavement areas (in orange in your screenshot) will be planted in grass. Apparently that didn't happen.

Re: environmental effects of removing blast-pad pavement, wouldn't the increased soil erosion from the shortened blast pads more than offset any improvement in drainage?

@randomhead. Thanks, I added this into the answer. For the reasons, I guess the whole section 5 list the criteria to be satisfied according to multiple national and local regulations, They are not so clear for me. Looks like the result of hard negotiations, and concessions  to get the project accepted by all stakeholders :-)

@Vkki: See my answer to @'randomhead. I really don't know. I read about grass being now replaced by artificial turf or paint to prevent tortoises to take residence...

"Airbus A380 and Boeing 787-8" - Did you mean 777-8?  The 787-8 is smaller and lighter than many preexisting widebodies.

@Vikki: Typo, it was 747-8. 787 and 777 are a cat V.

If the pavement painting was done to create a no-taxi area... what purpose, exactly, does that serve that wasn't already performed by the prior yellow chevrons?

@Vikki: The blast areas that long were not required, removal was to balance added pavement: "These areas would be planted with grass to offset grassland lost through the proposed airfield improvements." (see project details). But instead of grass they painted the old pavement for some reason.


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Why are only two of the five water-facing runway ends at KBOS equipped with EMAS beds?
KBOS has eight major runway ends:1 04L, 04R, 09, 15R, 22L, 22R, 27, and 33L.
As KBOS is primarily situated on (and occupies the vast majority of) a peninsula made by dumping lots of sand and dirt into Boston Harbor, and since making new land (especially new land that's going to be left completely unused except on the rare occasions that an aircraft goes off the end of a runway) is expensive[citation needed], only three of said eight major runway ends (22R and 27, which stop well short of the edge of the land,2 and 33L, which runs back up along the neck of the peninsula) are followed by full-size runway safety areas (RSAs):

(via Google Earth) Yellow denotes RSA without EMAS; green denotes EMAS beds; red denotes neither.
KBOS's other major runway ends (04L, 04R, 09, 15R, and 22L) drop off into the ocean just past the end of the runway.  As running off the end of a runway into the ocean is generally bad for non-seaplane aircraft[citation needed], this lack of RSA space would generally (in the U.S., at least) call for the installation of EMAS beds to catch aircraft overrunning these runway ends.  However, only two of the five aforementioned runway ends (04L and 15R) are actually equipped with EMAS; runways 04R, 09, and 22L each have only a short, un-EMASed blast pad separating the runway end from the dropoff into the harbour.
This is especially worrying when one considers that 04R and 22L are two of KBOS's five primary instrument runways (and 04R is one of the airport's two ILS-III runways, the other being 33L) - i.e., two of the runways that the airport preferentially uses in the very weather conditions that lend themselves to runway overruns!
Why are only two of the five water-facing runway ends at KBOS equipped with EMAS beds to keep them from discharging aircraft into the sea?

1: All references here to "runway ends" refer specifically to the departure end of the runway(s) in question, since that's the end that aircraft tend to run off of.
2: And whose RSAs cross over each other not far past the ends of their respective runways, as seen here.

Note that "runway safety area" refers to the protected area on either side of a runway as well as on each end. See AC 150/5300-13A for more information (check out paragraph 307 to start).

@mins: According to the FAA Airport Master Record for KBOS, runways 04R and 22L each have shorter LDAs (8851 and 8806 feet, respectively) than 15R (9202 feet), and 04R goes right up almost to the edge of the water like 15R does.  Yet, 15R has an EMAS, while neither 04R nor 22L does.  And all of these are among KBOS's primary instrument runways - in fact, 04R is one of only two ILS-III runways at the airport (the other being 33L), meaning that, in the very worst weather conditions, one of the two available runways will be one (1/2)

(2/2) ending in front of a drop-off into the harbour with no EMAS and hardly any RSA to speak of.

EMAS for RWY 27 end is planned for 2025/2026. Logan Runway 27 Runway Safety Area (RSA) Enhancements Project. Overall, interesting things in Logan 2018/2019 Environmental Data Report.

I'm not entirely convinced that either of your [Citation needed] notes actually need a citation.

@FreeMan No, but they’re amusing. XKCD does that a lot.

@mins: Judging from the map, there are at least three major-runway departure-ends at KBOS that need EMAS more urgently than 27 does...


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Why are the red/white control surfaces under the wing extended during a take off?
These pictures are more than a little dated as the CF-101 Voodoo's have not been flown since the mid 80's.  In these two photos we can see the Voodoo is performing a full afterburner take off.  Note the inside red coloured control surface at the back underside of the wing.  Why is this surface deployed like this?

and a moment later airborne

The airbrake flap are on the fuselage just aft of the tip of the red and white lightning bolt.  They are closed which I would fully expect on a take off.  Also the drag chute is not deployed so also another indicator that this is a take off and not a landing.
Here is a picture of a model that shows the control surfaces a little clearer along with the air brakes deployed.

What is this red control surface under the rear aft of the wing called and why is it deployed for a take off?
Note I am not a pilot, nor have I studied aerodynamics much beyond lift, thrust, gravity and drag.

I'm not an expert, but from a bit of googling, they seem to just be this plane's flaps (albeit a bit odd-looking). If so, they would be deployed to provide extra lift (at the cost of increased drag, so once you're airborne they would be retracted). They're detailed in this page, which is a review of a scale model of this plane — not inherently authoritative, but the author seems to know their stuff!

It looks like a slipt-flap. This kind of flaps were used because of their simplicity and almost no change in pitching moment when deployed, albeit at the expense of much more drag. If someone else confirms it, I could write it an answer with a couple of plots.

@sophit The page I mentioned in my comment above identifies it as a zap flap, fwiw.

You can see a detailed picture here: http://109lair.hobbyvista.com/DETAILSITE/US/USAF/f101/101a.port.flap.jpg

@yshavit: ah ok, never heard about them but I suppose they work just like split-flaps, i.e. useful when a change in pitching moment as small as possible is needed

@yshavit: in case I'll come up with an answer, may I use the link you provided regarding the zap-flap?

@sophit Of course! I am off to bed now, but I just got these links from a bit of googling, so I don't have any special claim to them. :-)

"Also the drag chute is not deployed so also another indicator that this is a take off and not a landing."      Pretty sure the afterburner should be the dead giveaway that this is a takeoff :)

These are not control surfaces, but a special type of split flap. In a split flap, the rear part (20% to 30%) of the lower wing panels is hinged at the front and can be lowered, thus increasing wing camber.

In case of the F-101, the hinge point moves aft in addition to the flap being lowered, thus increasing wing chord a bit. This type of split flap is called a Zap flap after its inventor, Edward F. Zaparka (invented in 1932).

A split flap gives the highest lift increase for a simple mechanism, but also increases drag considerably. Therefore, it is best suited for landing. For takeoff, it  should be opened only partially (10° to 20°) but for landing 45° to 60° are common.

As per link from @yshavit it is actually a zap-flap, very similar but it moves backward as well slightly extending the chord/surface

One interesting thing about the CF-101's flaps is that they seem to move not exactly backwards, but back-and-out (ie, they're hinged almost on a pivot, relative to the fuselage). You can see that from an alternate view here. I wonder if that was for aerodynamic reasons, or for something else, like to accommodate some armament. The CF-101 could fly with two external fuel tanks, so maybe this was a way of directing air away from them?

I read somewhere that the F-101 only had two flap positions: UP and 45 degrees which was used for both takeoff and landing.

@yshavit -- re "One interesting thing about the CF-101's flaps is that they seem to move not exactly backwards, but back-and-out (ie, they're hinged almost on a pivot, relative to the fuselage)."-- based on this pic, http://109lair.hobbyvista.com/DETAILSITE/US/USAF/f101/101a.port.flap.jpg , my guess is that the hingeline of the flaps is simply nearly spanwise, as opposed to the marked forward sweep of the inboard trailing edge.  So the flaps are much wider inboard than outboard.  I think this accounts for all the apparent weirdness that we see in the pics.

PS your own photo link seems to show same.  (PPS A strongly forward-swept hingeline would cause the flaps to create a powerful anhedral effect in terms of slip-roll coupling, when deployed, which might not be a good thing.)

@quietflyer I think you're right! Thanks.

At a first sight it looked like a split flap but as @yshavit found out, it is actually a zap flap. A zap flap is a particular kind of split flap that via a leverage moves not only downward but also backward, increasing the chord as well. Apparently the name derives from his prolific inventor, Edward Frank Zaparka, who patented it in the 1933.
Split (or zap) flap together with plain flap have been historically the first iteration of trailing-edge high-lift devices. They have been superseded by more efficient albeit mechanically more complicated flaps, like the three-slotted flaps seen on virtually all modern jetliners. This picture taken from Wikipedia shows the several types of existing flaps.

Why a zap flap instead of a simpler split flap? The following plots taken from this NACA TN show that moving the flap backward as well, slightly increases the $C_{l_{max}}$ practically without any increase in the $C_d$:

On the left side there's a split flap while on the right side there's a zap flap.  Due to the backward extension of the flap, the chord i.e.wing surface becomes bigger and  bigger surface implies higher lift.
From the plots it can also be seen what is in general the effect of a split (zap) flap: both the plots of $C_l$ and $C_d$ are basically shifted more and more upward the bigger the flap deflection is. With modern slotted flaps the upward shift of $C_d$ is more limited.
But why exactly a zap flap instead of a plain or nother kind of flap? According to the relevant wiki entry, the F-101 suffered from "severe pitch-up at high angle of attack". High angle of attack are seen also during landing phase so a kind of flap which didn't changed too much the wing's pitching moment would have been welcomed: split flaps have just this characteristic. So it might be (just speculating here) that the choice of a split (zap) flap was dictated by this requirement.

I think that is a flap used on takeoff and landing. They are normally deployed (partially) on takeoff and  (full) on landing. Flaps help in increasing lift at slower speeds. They are called 'high lift devices'. You can see them here(landing)

In your picture all 3 sections extended downward compose the flap, not only the last one with the arrow

@sophit  i know i will edit the picture

Note that the flaps in the picture are much more complex than the flaps on the fighter. I assume that is a deliberate tradeoff: flaps on airliners are designed for efficiency and safety, whereas the fighter values space, weight, and simplicity and pays for that with increased drag … which is much less of a problem for a fighter, because they tend to be quite overpowered compared to an airliner.


text:
Did the DC-9 ever use RATO in revenue service?
While reading the NTSB report on the crash of ALM Flight 980, I came across this interesting tidbit:

...  The fuel charts used for the study made allowance for a higher consumption rate than the standard DC-9-30 series aircraft.  This was the result of a fuel audit by the Douglas Aircraft Company on ONA aircraft and was due to jet assisted takeoff fairings installed on those aircraft.  [Pages 7-8 of the paper report; pages 12-13 of the PDF file of the report.  ONA = Overseas National Airways, who operated the flight on behalf of ALM.  Emphasis added.]

Did these DC-9s ever actually use RATO during revenue operations?
EDIT: "Revenue operations" here means "any flight conducted for monetary compensation".

The JATO/RATO retrofits were used only by Overseas National (later National Airways) when operating out of hot/high airfields for military contracts. They were never used as passenger flights, but you could say that these were "revenue" flights as they were done for monetary compensation.
I can't find any history on ALM980, only that it was registered as N935F and operated by Overseas National as a passenger flight. N935F does not show any history in the FAA registry other than it being reserved. It is possible that this was a converted cargo aircraft.
Here is a picture of an Overseas National flight using the JATO rockets:

Source: UT Dallas
It should be noted that the DC9 was not the only passenger type aircraft to get the "JATO" treatment. Boeing modified the 727 to operate out of short fields by adding JATO rockets. Aviation Week did an article featuring the DC-9 JATO modifications:

Source: Aviation Week (article paywall)
The magazine cover linked above shows a DC-9 take-off (an Overseas National flight).


text:
C172 models and panels design
How many C172 models there are out there and what are the differences between them?
Which models have these "modern" (newer) instruments panel design like this one:

versus:

I'm not sure what you're asking here. Even two C172s from the same model year can have completely different panels depending on what the buyer ordered, or later owners upgraded. And FWIW, the panel in your picture doesn't look very modern to me; I don't see any GPS-based navigation or 'glass' instruments. C172s have had the Garmin G1000 available since 2006 or so. Perhaps you can be more specific about what information you're looking for?

@Pondlife That looks like a C172R/S, which originally came standard with a KLN89 GPS, as in the picture just below the audio panel. The metal panel itself, more than the instruments and avionics, was "modern" in an aesthetic sense vs earlier models.

@StephenS Good catch! Whether or not a KLN89 is "modern" or not is still debatable, I guess :-)

If you are asking “When did the Cessna 172s adopt the modern 6-pack layout, the answer is 1968. You can verify this by looking at for sale pictures. All of the 1968 panels have the normal 6-pack layout while very few of the 1967 do. The revised layout in the 1967s is obviously not from the factory. They aren’t completely lined up like in your picture for a few more years. If you care to know when, browse through some for sale listings.
Cessna 172 - 1967

Cessna 172 - 1968

Also,I have a copy of the 1967 Owner’s manual and it shows the old layout like in the first picture.

Those aren't new panels. The latest are like this.
I think the other answers are already heading towards "When did 6-pack for IFR and Radios/GPS/etc end up the center stack?" Things like Intercom, Nav/Com, ADF, LORAN, transponder, GPS with 2-line display, then GPS controlling external radio (Northstar, nice radio too), then GPS/Moving Map, then GPS/NavCom/moving map, then Flight Management System (latest Avidyne units, perhaps the Garmins too, showing more & more stuff - ADS-B In, fuel info, approach charts, selectable data on nav charts, synthetic vision, other engine instruments), transponders with ADS-B Out and Out/In feeding the FMS for display,
Standalone autopilot, autopilot coupled to ILS, coupled to GPS, coupled to fly approaches, etc. It just goes on and on.

The first picture appears to be of an R or S model, produced since 1996. The second appears to be a M, N or P model, produced from 1973 to 1986.
The two panels in your picture are nearly identical; they have all the same instruments and controls in the same locations. The avionics varied slightly due to what was current at the time, but that doesn't seem to be what you're asking about.
The most obvious difference between the two is the material and color of the panel itself: the R/S models use gray metal, whereas the older ones used black Bakelite (plastic). This is an aesthetic difference, not a functional one, and has no practidal effect other than a minor difference in weight. Older planes can be (and often are) easily upgraded to metal panels to get the more modern look.


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Why would a PAPI and ILS glide slope for the same runway not coincide?
Some pilots are reporting the glide slope on our PAPI is not coinciding with the glide slope on our ILS even though FAA did a flight check of our PAPI's. Are there any reasons why a PAPI and ILS would indicate different approach glide slopes?

Which airport are you referring to?

From the FAA AIM (section 2-1-2(b) Visual Glideslope Indicators): PAPI angles are based on obstruction clearance +/- 10 degrees from the runway centerline, up to 4 miles from the threshold. The ILS provides for obstruction clearance in a 3-6 degree angle on a 3 degree glide path. Therefore, if there are obstructions in the PAPI clearance area (e.g. a tower off to the side of the rwy) that don't exist in the narrower ILS clearance area, the PAPI will have to be a steeper angle.

As a simple word: Height of eyes vs height of ILS antenna.
The extract from Transport Canada is as follow,

However, this is not possible to achieve completely because the pilot eye is above the aircraft antenna which senses the ILS signal and the angle "B" of the lower edge of the PAPI approach corridor is less than the angle of the ILS signal.

They are not always coincident either.

At various airports e.g Paris De Gaulle (Lfpg), the PAPI's are set for cat D I.e B747, so the medium to light A/c class, will on a Cat 1 approach, appear to below PAPI 3% glide angle.

Pilot complaints about mismatch between ILS and PAPI information are very frequent, and not necessarily a sign of malfunction.
There can be many factors at play, but assuming good design and good operating condition of both systems, this mismatch is due to the differences in aircraft geometry amongst aircraft types, and more specifically due to where the Glide Path (GP) aerial antenna is located with respect to the cockpit for each type of aircraft.
The Glide Path signal is perceived by the antenna, which is usually on the nose of the aircraft, whereas the PAPI, being a visual aid, is perceived by the pilot's eyes. Each aircraft type will have a different location of this antenna with respect to the cockpit and the eyes of the pilot, but they are never at the same height. This causes misharmonization of the ILS and the PAPI.

PAPI siting, for an ILS runway, is done such that the PAPI provides as close signal as possible to the GP signal for the majority of aircraft types that use that particular runway.This process is called harmonization, and by definition the degree of harmonization will differ for each aircraft type.
Pilots will therefore perceive that the information visually acquired from the PAPIs will differ from the on-path information obtained from their on-board instruments. This is normal and to be expected at any airport where PAPIs are used in conjunction with ILS, and it is more accentuated at airports where there is a wide range of aircraft commonly using the runway.
That said, a good harmonization will aim to err on the side of safety, meaning it should never give a fly-down (three whites and one red) due to lack of harmonization for a given aircraft type, but a fly-up signal.

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